Coalification history of the Stephanian Ciñera-Matallana pull-apart basin, NW Spain: Combining anisotropy of vitrinite reflectance and thermal modelling

Abstract

The Stephanian Ciaeera-Matallana Basin of NW Spain comprises 1,500 m of alluvial to lacustrine coalbearing sediments, which were deposited in a late Variscan transtensional/transpressional pull-apart setting. The relationship between coalification pattern and rock deformation was evaluated by measurements of the anisotropy of vitrinite reflectance (AVR). The AVR ellipsoids reveal both pre-tectonic elements related to the bedding fabric and syn-tectonic elements related to folding, producing biaxial ellipsoid shapes with the maximum reflectance parallel to fold axes. The mean coalification gradient for the Stephanian succession is about 0.62 %Rr/km. Calculations of the mean palaeogeothermal gradient are presented on the basis of three different empirical equations. A palaeo-geothermal gradient of 85 C/km is considered the most realistic, with an overburden of about 1,000 m. 1-D numerical modelling of the burial history results in two possible scenarios, the most preferable involving a palaeo-heat flow of 150 mW/ m 2 and an overburden of ca. 1,050 m. These results indicate that maximum coalification was related to a localised but high palaeo-heat flow/-geothermal gradient. The anisotropy of vitrinite reflectance highlights the interactive and transitional nature of sedimentary compaction and rock deformation on the maturation of organic material within strike-slip fault zones.

Thermal history of the Sabero Coalfield (Southern Cantabrian Zone, NW Spain) as revealed by apatite fission track analyses from tonstein horizons: implications for timing of coalification

“…3.96 % Rmax) in rare cases, mostly in close association with sills (Botor et al 2009). Estimated geothermal gradients of 52 °C/km (Botor 2012) are lower than those at other Stephanian basins (Mendez 1985;Frings et al 2004;Ayllon et al 2003) and in the Palaeozoic basement (c. 35 °C/km; Brime et al 2001). Aller et al (2005) argued that increased heat flow due to hydrothermal-fluid circulation was the last important thermal episode in the SCZ, facilitated by pathways along faults.…”

“…Therefore, new models (Fig. 3) were constructed taking into account previous thermal history studies in the SCZ (García-López et al 1997, 2007Ayllon 2003;Ayllon et al 2003;Frings et al 2004;Aller et al 2005;Frings and Warr 2012;Botor 2012), which indicated that the Late Carboniferous-Permian period was the main heating time. Vitrinite reflectance data were used to calculate the maximum palaeotemperatures (values T max 2 and T max 3 in Fig.…”

“…As the Stephanian sedimentary rocks in the basins generally have a higher coal rank than older Palaeozoic sediments in the same region, it has been suggested that the local magmatism (sills and dykes, volcanism) increased thermal maturation (Aller et al 2005;Colmenero et al 2008), producing a coal rank of up to anthracite level (above 2.0 % R r ). In the Cinera-Matallana Coalfield (14 km west of the Sabero Coalfield), where the coal ranks are higher (R r up to 2.6 %; R r gradients are also higher), the average palaeogeothermal gradient during the Early Permian has been calculated as 85 °C/km with a heat flow of 150 mW/m 2 (Frings et al 2004), although Mendez (1985) estimated a lower palaeogeothermal gradient of 60-70 °C/km, similar to other Stephanian coal basins across the Cantabrian Zone . Heat transfer by magmatism and related hydrothermal activity could have resulted in very high geothermal gradients and heat flow values (even over 100-160 mW/m 2 ), as seen in the coalfields of the Massif Central, France (Copard et al 2000), Cinera-Matallana Coalfield, Spain (Frings et al 2004), Bohai Basin, China (Hu et al 2001), or the eastern Alps (Sachsenhofer 2001).…”

“…In the Cinera-Matallana Coalfield (14 km west of the Sabero Coalfield), where the coal ranks are higher (R r up to 2.6 %; R r gradients are also higher), the average palaeogeothermal gradient during the Early Permian has been calculated as 85 °C/km with a heat flow of 150 mW/m 2 (Frings et al 2004), although Mendez (1985) estimated a lower palaeogeothermal gradient of 60-70 °C/km, similar to other Stephanian coal basins across the Cantabrian Zone . Heat transfer by magmatism and related hydrothermal activity could have resulted in very high geothermal gradients and heat flow values (even over 100-160 mW/m 2 ), as seen in the coalfields of the Massif Central, France (Copard et al 2000), Cinera-Matallana Coalfield, Spain (Frings et al 2004), Bohai Basin, China (Hu et al 2001), or the eastern Alps (Sachsenhofer 2001). Recent work on clay minerals in mudstones from the Cinera-Matallana Coalfield (Frings and Warr 2012) has revealed a complex mineralization history related to localized igneous and hydrothermal activity associated with strike-slip faulting.…”

“…The porosity and degree of compaction of the Stephanian sediments also likely to have played an important role in the coalification process, since relatively uncompacted and uncemented sediments would have facilitated fluid circulation (Duddy et al 1994). Therefore, any related fluid activity that may reflect anomalous thermal gradients can be expected to have left an imprint upon the Stephanian clastic deposits and coals, either during or after their deposition (Frings et al 2004;Botor 2012). Moreover, the abundance of hydrothermal mineralization and ore deposits in the Palaeozoic rocks (mainly in Carboniferous) related to regional faults constitutes the important metallogenic feature of the SCZ (Luque et al 1990).…”